Hmm, looking through the units.txt entry for this thing, it has (some deleted)
BombardRange 1
CanCounterBombard
ActiveDefenseRange 1
Explodes [???]
CanAttack: Air
CanAttack: Land
CanBombard: Land
CanBombard: Mountain
CanBombard: Water
CanBombard: Air
Defend: Air[/list]
From this, I would read that it should only actively defend against Air (not Water). Maybe try editing this to include Defend: Water and Defend: Land.
See if that helps
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